It turns out the local BMW indy I talked to was right about the ATF being low. He recommended I use the following procedure to top it up:
- Loosen the fill plug to where it just barely prevents the ATF from leaking out
- Start the car, dive under the car, remove the plug as quickly as possible, and start pumping fresh ATF as fast as possible. If you wait too long, the fluid flowing in the transmission will start to start frothing over and out of the fill hole due to cavitation.
- As soon as a significant amount of fluid starts spilling out of the hole, stop pumping, and put the plug back as quickly as possible.
While this procedure flies in the face of the accepted methods, I think it actually makes sense, because I measured how much I drained out with I did the drain & fill in June of last year, and how much I put back in, and there was a difference of about half a quart. This puzzled me, but I figured I must have made a mistake measuring. Now what I think happened is a combination of two things: 1) I think I got the fluid too hot during the procedure, causing it to expand, and appear more full than it actually was, and 2) I suspect I had the car at an angle, with the left side lower than the right.
I added just under 3/4 of a quart this time, which is close to the discrepancy when I did it last year. Is this enough to fix the slipping symptom? It didn't do it when I tested it this morning after topping up the fluid, but I'll have to wait for it to cool off (going to be a balmy 82 degrees again today) to know for sure. Fingers crossed...